In ultrasonic imaging systems, an acoustic signal is transmitted by a transducer element to the object being imaged and the echo of the acoustic signal bouncing off such object is detected by the transducer and used in a well known manner to produce an image of the object. In order to propagate the acoustic signal and to minimize echoes at the point where the acoustic signal enters the object being imaged, the transducer is typically immersed in a fluid having an acoustic impedance which substantially matches that of the object. Thus, where such transducers are being utilized to do ultrasonic medical imaging, the fluid utilized in such transducers has an acoustic impedance which substantially matches that of human body tissue. While the portion of the housing in which the fluid is encased, and through which the ultrasonic signal is projected, can be formed of a material, such as various plastics, which has an acoustic impedance which does not substantially differ from that of the fluid, the fact that the acoustic impedance of the plastic or other material through which the signal is projected does not exactly match that of the fluid results in a certain portion of the acoustic signal being reflected back into the fluid at the fluid/plastic junction. The percentage of the signal being reflected back at the fluid/plastic junction is typically quite small, for example approximately 5% of the acoustic signal. While some of this signal is immediately reflected back to the transducer, and can be ignored by the imaging circuitry, depending on the angle at which the acoustic signal is being transmitted, varying portions of the reflected signal bounce off other elements either in or forming the walls of the chamber encasing the transducer fluid. Some of these elements are formed of material such as aluminum or other metal having an acoustic impedance substantially different from that of the fluid, resulting in significant reverberation echoes being formed at the junction between the fluid and such material. Since the sensing elements utilized in such transducers are extremely sensitive, they pick up such reverberation echoes. This results in unwanted echoes or other spurious elements, which spurious elements can interfere with the intended use of the image.
Heretofore, an effective technology has not existed for economically dealing with such reverberation echoes, particularly for relatively small transducers, such as those used for medical imaging, where there is little space for damping elements. Most transducers have merely accepted the spurious elements caused by such reverberation echoes and no effort has been made to damp them. To the extent any effort has been made to deal with the problem, such efforts involved placing material having a high acoustic-attenuation characteristic at the junction between the fluid and the metal or other material causing the reverberation echo. However, since the acoustic impedance of such materials also differs substantially from that of the fluid, reverberation echoes also form at the junction of such material with the fluid, resulting in limited improvement in the image provided by the transducer.
Another problem with using esisting attenuators is that, to the extent such materials are porous and/or have irregular surfaces, they may trap air which may get into the fluid filled chamber. Since air is a perfect reflector, any trapped air, particularly air trapped at the surface of the attenuator, can result in the attenuator enhancing rather than reducing the reverberation echo effect.
It is therefore a primary object of this invention to eliminate, or at least substantially reduce, spurious images caused by reverberation echoes in acoustic transducers.
A more general object of this invention is to provide an acoustic damper capable of substantially eliminating reverberation echoes at the junction between a fluid and a material having an acoustic impedance substantially different from that of the fluid.
Another object of this invention is to provide an acoustic damper of the type indicated above which is suitable for use in small transducers such as those used for medical imaging.
Still another object of this invention is to provide an acoustic damper of the type indicated above which does not trap or absorb air and therefore remains acoustically stable.